1. Field of the Invention
The present invention relates to elastic surface wave force sensors and more particularly to those comprising at least two oscillators connected to transducer means situated on both faces of a resilient plate bending under the action of the acceleration. It may more particularly be used in accelerometers and in this case their structure allows the acceleration to be measured along a component of direction normal to the main faces of the plate.
2. Description of the Prior Art
Such a plate generally comprises at least one end embedded in a frame and a mass acting on the plate in said direction. The transducer means are designed for exciting and collecting elastic surface waves whose propagation speed is known to vary as a function of the mechanical bending stresses. In a first embodiment, the transducer means form a transmission line whose delay time determines the phase shift of an oscillating loop. In a second embodiment, the transducer means are situated in a resonating network cavity and are connected to means maintaining the oscillation of the cavity. The frequency representative of the acceleration to be measured results from the subtraction of two oscillation frequencies which have thermal drifts which do not exactly counterbalance each other.
In the design of such devices, it is necessary to provide plates having a geometry such that they provide zones of equal resistance corresponding to the measurement zones so that, during deformation of the plate, the stresses in these zones are uniform and cause a uniform speed of propagation of the elastic surface waves. Moreover, these devices may be designed so as to overcome the thermal drifts created by the stresses at the embedment points of the plate.